My PC evocation wizard is trying to learn some dunamancy spells in game. The spells are not currently listed on his list of spells he can learn. After messing around in the character creator I found that wizards are currently unable to learn dunamancy spells unless they are the proper subclass (ie. cannot learn graviturgy unless graviturgy subclass, or chronurgy unless chronurgy subclass). In EGW, dunamancy is supposed to be rare, but accessible by other wizard subclasses to learn from dunamancy wizards.

Uncertain if this exclusivity is intentional or a bug?

It is intentional - because those spells are not on the Wizard's spell list - they are their own separate spell lists. So normal Wizards don't have access to them.
A DM can allow them - but that would be an optional rule and DNDBeyond doesn't really support optional rules. You would need to use the homebrew tools in order to get your Evocation Wizard able to learn them.

Ok. Thank you. I see what you’re saying, but right now, graviturgy/chronurgy wizards don’t even have access to all the dunamancy spells, only the specific subclasses spells. Perhaps I misunderstood the EGW about all dunamancy spells being available to both subclasses?

Some of the Dunamancy spells are absolutely specific to one or the other subclass.
If you look at the Dunamancy Spells page - those marked with a * are Chronurgy and those with ** are Graviturgy - those without are available to both.

Ah, thank you. I missed that before.