Have not had this happen on my PT945. Could be a faulty Slidestop Lever Return Spring which is easily checked.
1.) Check to make sure your Pistol is unloaded before proceeding.
2.) Field Strip Pistol, removing the Slide/Barrel assembly from the Frame
3.) Holding Pistol in your right hand Turn Pistol sideways and push Slidestop Lever up with your right hand thumb
4.) Remove your thumb and check to see that the lever springs back to the level position. If not, the Slidestop Lever Return Spring is broken and must be replaced. It is not difficult to remove and install. (At least from my point of view.)
Taurus will send you a New Slidestop Lever Return Spring, but you may have to buy it.
Since you do not know how many rounds have been thru your pistol, a new recoil spring might be a good idea, as well.
The only other causes I can think of may be that you are accidently brushing up against the Slidestop Lever during your shooting or possibly LimpWristing the Pistol during shooting. Have someone else shoot your pistol and see if it happens to them.
WinClean has caused some other problems in the past. Please try some different ammunition, as well.
I also second the idea of Bumper Pads to protect the Plastic Bottoms of your magazines. If you are handy these can be fabricated from generic Black Mouse pads and glued (I use Goop silicone glue) to the bottoms of your magazines. Just be careful to leave a hole for the disassembly button of your magazines.