Is this with an escort order on the combat ship?
Carriers auto-escort nearby combat ships so they "hide" behind them and are generally in a better position to support them, since if they're far away, it takes longer for each attack run.
Hmm - in the dev version, I've already changed it so they won't try to escort ships one size smaller, since, yeah, that can lead to problems. As you noted, though, a frigate will often get enough separation so it's not very noticeable in the first place. A faster ship of the same size is more questionable; I think it can be beneficial and I'm not sure the cases where it causes problems are that much of an issue in an actual fleet fight - the faster DE will separate, and the carrier should find another nearby ship to hide behind, or just back off... or the faster DE will become part of a battle line, where its speed doesn't factor in for this, and the carrier being nearby-ish is helpful. So the window for when this could be bad seems fairly narrow (and, granted, probably most visible in a simulator setup). It's not very clear where to draw the line.
Some ships (Mora, Legion) will also no longer do the auto-escort behavior - not 100% sure if that's already the case of if it's just an in-dev thing.