Known issue, but the devs have stated that they don't really have plans to cater to having 20+ fleet ship types unlocked at once, so its not that high priority at the moment.
If they decide that 20+ ships goes from not recommended to not supported, then the number of homeworlds one player can have should be capped at 12 (which would mean 20 ship types total. 3 for the triangle ships, 12 from your homeworlds, and 5 ARSs), thus making this impossible to trigger, and showing the player explicitly that ludicrous numbers of bonus ship types are not supported. (Actually, it would be less than twelve, due to the science lab having a similar restriction, and the science labs have additional entries on their fleet ship Mk. unlock tab)
Whether they put a hard cap or not, IMO, there should be some way for the build tabs to support >20 columns, in case it is needed in the future, plus just as good "bulletproofing". (It doesn't always have to be all there at once, some sort of horizontal scroll bar or something will do)