I start with an artificer at level 1 and have 4 prepared artificer spells. I then take a level of wizard and I now am only allowed 3 prepared artificer spells. ie I get an error saying I have 4 of 3 prepared slots.

Is this intended? I know how multiclass spell slots work but I didn’t expect this on prepared spells being “unprepared”.

Have you been changing your intelligence

Your Artificer prepared spells should be you int mod + half your Artificer level rounded down.

At level 1 you should only have 4 spells prepared if your int was 18 or 19, was this the case?  At Artificer 1 wizard 1 your Artificer spells prepared does not change.

I suspect you set 4 prepared spells with your int at 18 and then decided to make some changes that reduced your spells prepared.

Yes it was my intelligence - good thought.

the free feat i had was switched to no selection when I do a “Copy of” which certainly wasn’t expected behaviour/some form of beyond bug but at least I can fix now I know what it is and that it isn’t a rule I wasn’t aware of.

The intent is to leave artificer spells unchanged and to add wizard spells equal to your int mod + your wizard level.

One of the real advantages to multi-classing is a large increase in the total number of prepared spells.

Often you end up preparing 0 or 1 of first level spells of your 'main' class, instead filling it with your less class.