Hello Chad,
As APDL doesn't use "A/C Types" but only tail numbers (Idents) it has to lookup prior entries in your logbook to see if the incoming ident has been used previously and then it will pull the corresponding A/C Type from that search, otherwise fall back to the Default A/C Type setting. The only way to "remove" an Ident is to change it where it was used in the past. You can sort the idents to help find them by clicking the column header in the spreadsheet log area, click the column header again to sort opposite. I'll make a note in our tracker that we are sorting our lookup so that we find the most recent use of an Ident and don't end up finding an older entry where this could cause a problem such as you describe. Once you make the correction in the later flight entries it should then correct itself automatically.