spells still rolling with 5e rules not not 5.5e rules

Last night I used a spell I had not used before which was Blades of Disaster. When I used the spell last night, it rolled with the Forgotten Realms: Heroes of Faerun number of die for a normal hit. However later I crit on an 18 and it rolled the normal damage listed for the new rules (10d6) but it also rolled 8d12 extra damage for the crit which is from the old Tasha's rules and not the new Forgotten Realms: Heroes of Faerun rules which does not mention 8d12 force. I am playing a 5.5e character and had to reroll the extra damage as it rolled incorretly in DND Beyond. I am trying to report this, but I don't know whom to contact or if it will ever get changed. Please Help.

Tasha's: 

You create a blade-shaped planar rift about 3 feet long in an unoccupied space you can see within range. The blade lasts for the duration. When you cast this spell, you can make up to two melee spell attacks with the blade, each one against a creature, loose object, or structure within 5 feet of the blade. On a hit, the target takes 4d12 force damage. This attack scores a critical hit if the number on the d20 is 18 or higher. On a critical hit, the blade deals an extra 8d12 force damage (for a total of 12d12 force damage).

As a bonus action on your turn, you can move the blade up to 30 feet to an unoccupied space you can see and then make up to two melee spell attacks with it again.

The blade can harmlessly pass through any barrier, including a wall of force.

Forgotten Realms: Heroes of Faerun:

You create a 3-foot-long blade-shaped planar rift that lasts for the duration. The rift appears within range in a space of your choice, and you can immediately make up to two melee spell attacks, each one against a creature or object within 5 feet of the rift. On a hit, the target takes 10d6 Force damage. This attack scores a Critical Hit if the number on the d20 is 18 or higher.

As a Bonus Action on your later turns, you can move the rift up to 60 feet and repeat the two attacks against a creature or an object within 5 feet of it. You can direct the attacks at the same target or at different ones.

The blade can harmlessly pass through any barrier, including ones created by spells like Wall of Force.

Is this in the app or website? 

The only thing I can think of is if you are using an extension like Beyond20 but that is for a browser not the app. The reason I thought that is because you mention the roll automatically rolling crit damage on an 18 but this isn't something D&D Beyond can do. It can only register crits on a 20 and if you crit on a lower number due to a spell or effect you must manually make it roll critical damage. I did test this both on site and in app by adding both version of the spell and rolling under I got an 18 on the die - which did not adjust the damage to a crit - and then rolling until a got a 20 on the die which did adjust the damage die (4d12 went to 8d12 for 5e version, 10d6 became 20d6 for the 5.5e version). 

Other than "double dice for crit" D&D Beyond has no automatic rolling of additional damage dice. For instance even scoring a Nat 20 crit on the 5e version of the spell it only rolled the 8d12 (the double dice for crit) and did not roll the extra 4d12 (for the total 12d12 as detailed in the spell). 

The Beyond20 extension does "support" extra damage dice auto rolling and enhanced crits. However, it is very buggy which is why I stopped using it quite some time ago.

So, since everything you describe is a common issue with the Beyond20 extension it made me think you're using that. But this forum is for the app, which you can't use browser extensions for, so I am curious if this is indeed an issue you have with the app or if you were using the site with the extension and just posted in the wrong place?

If you are using the app can you detail what device you're using? I only had access to my samsung/android phone and could not replicate the issues in the app.

If you are using the website but are not using Beyond20 - can you detail what browser you are using, any extensions or VPNs? I tested the site on windows 10, windows 11 and an xbox one using browsers Chrome, Firefox, Edge and Opera GX and could not replicate the issue.