When you perform a Live-To-PTR copy, you select a source region (i.e. US, EU or Asia) and it copies all of your heroes from that region to the PTR server. On the PTR, Season 25 has ended so all of your current, live seasonal heroes will become non-seasonal on the PTR. All the items that were in your seasonal Stash will be in your non-seasonal emails. Your non-seasonal and seasonal XP will be amalgamated and your non-seasonal paragon will be set to match the new XP total. If you want to test season-specific stuff, you’ll need to make a fresh seasonal hero on the PTR.
However, the live-to-PTR copy function is currently broken, and awaiting updates from Blizzard as to when that will be fixed.
So, if you go ahead and create a seasonal hero on the PTR now you need to be aware that if you do a live-to-PTR copy later (once they fix it) this actually overwrites whatever you have on the PTR with what’s being copied from live, i.e. it will overwrite any PTR seasonal heroes you’re currently working on.