In the meanwhile, I replaced the form, which functions as it should. However, I keep wondering how could this have happened!

Very strange. Sounds like you got some errant code in there somewhere. Many times I've found solutions by just scrapping a form and starting over. :)

You can use parts of the code below to add 1 to numbers.  I have cases for serial numbered letters with numbers that look like G-0789.  The code below uses DMax to find the last largest.  The number part is separated from the text part so numbers can be incremented by one, then recombined.  That code looks like:
DoCmd.Requery  'this helps with secondary additions while the form is still open.
DoCmd.GoToRecord , , acNewRec
Dim varResult As Variant
' in DMax(), use the FieldName and the TableName, respectively
varResult = DMax("GenOutSerialNum", "tblGenCorrOut")
' This next part separates (parses) the text from the number part,
' adds 1; then puts the parts back together with leading zeros.
Forms.frmGenCorrOut.[GenOutSerialNum] = Left(varResult, 2) & _
Format(Val(Right(varResult, 4)) + 1, "0000")


For our incoming correspondence, it looks like IC-45678
Me.[IC_series] = Left(varResult, 3) & _
Format(Val(Right(varResult, 5)) + 1, "00000")
DoCmd.RefreshRecord

Oh, this might help, too.  To help me save, I suppose any entries on the Incoming Correspondence Form, I see, years ago, I added this little note that seems to have been working for me since 2017.  A better solution is probably DoCmd.RunCommand acCmdSaveRecord.  The note to myself in 2017 that just I noticed in the code to create the G-0789 style of numbering after creating a incoming correspondence record looks like:
Private Sub cmdOpenGSeries_Click()
' RefreshRecord to write to table to avoid Dirty Record
' RefreshRecord does same here as SaveRecord.
' DoCmd.RefreshRecord
' 2017-07-22 To save the IC record, replace the command above with:
If Me.Dirty = True Then Me.Dirty = False

DoCmd.OpenForm "frmGenCorrOut", acNormal