I think my 6 second all out effort changed my Fitness Signature too much

Go ahead and flag the activity. See: Flagging Activities – Xert

It’s likely your PP and TP are understated but you are best off validating your signature with a Xert standard fitness test workout ridden in Slope mode. See: From old trainer to smart trainer. Set FTP - #5 by ridgerider2

Reference – Breaking Through the Xert Way! – Xert