The mechanism has a trip system that activates when the arm reaches the inside grooves.
Obviously, something is loose, misadjusted, or bent causing it to trip before it gets there.
There are many pieces to this, so if you are mechanically inclined you may be able to determine why it is tripping wrong by watching it.
[link|https://www.manualslib.com/manual/689429/Technics-Sl-1900-M.html|Here] is the service manual.
Remove the stylus before you start so you don't damage it.
You will have to get access to the bottom and maybe remove the platter and move it manually.
The platter should just lift off, so don't invert it before taking the platter off.