Ignore it. Start afresh with a new ‘16th’ Doctor. Then later on, throw in a line like -
‘The regeneration process is a bit messy. Sometimes I get sick or lose my memory. Once I turned into an old friend of mine for a few minutes before settling on my final form.’
Done.
The "16th Doctor"/PiperGate is going both remain an enigma and an incredibly unfortunate burden on the next showrunner to either sort out or ignore. Most likely ignore for the show's health but then do you start with a new "16th" or jump to "17th"? If so, WHAT happened there?