Yes, it sounds high but, I am confused, why was the drain "fixed" by someone else a few years ago and was that person licensed? You might as well move forward and fix the problem and forget the past.
The first thing I would do if the ceiling is made of drywall, is cut out the bad portion of the ceiling below the tub. You are going to have to do this anyway so you might as well do it now.
Do you have an access panel to get the the hot and cold water valves behind the wall where the shower is? If so, I would remove it.
Steve is right, it may be from another spot.
And that would make sense because this seems to be the third time since the tub was installed that you have had this leak.
It may be leaking from a bad valve!
So I would try this,
I would run the cold water in tub, not the shower, and look for leaks.(always work your way up when it comes to leaks and give it some time for the water to show up).
Next, I would run the hot water in tub, not the shower, and look for leaks.
Next I would run the shower and look from below to see if you can see where the water is coming from.
Water travels and will seek the lowest point, or the path of least resistance.
If you can locate the leak, you should call a licensed plumber to repair it! Ask your neighbors or friends at work for referals.
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