Different problem I suspect brother_hesekiel.

OP referred specifically to Classical guitar and mentions using a chair and guitar footrest, so in the absence of clarification it would be reasonable to assume he is probably learning that style using Classical positioning convention for holding his guitar? Or not???

As in your case with the bar stool, holding in the more common style I experience the same problem when seated on one of those rotating round seat bar stool with ring footrest type sold in guitar shops emblazoned with various guitar and gear logo brandings. Tried everything. Foot placement and angle when seated on them, but the angle of the thigh inevitably results in the guitar to slipping away, even with a conventional body acoustic. Electric is worse. Might work for someone 6'4" with proportionately long legs, but not for me. I have to use a strap to play on the bar stool.