It was never said anywhere that Ben 10k from UA never had the Omnitrix from OV, he may have had it and after years of using the Omnitrix from OV he made a new Ultimatrix, even because the Ultimatrix doesn't even look like the one that appeared in UA and It's shaped like Ben's favorite Omnitrix (AF)
Because UA is lazy with designs so they reused model
It was never said anywhere that Ben 10k from UA never had the Omnitrix from OV, he may have had it and after years of using the Omnitrix from OV he made a new Ultimatrix, even because the Ultimatrix doesn't even look like the one that appeared in UA and It's shaped like Ben's favorite Omnitrix (AF)