It's easy to put the cart before the horse, sometimes.

If the adjustment works fine for a time after leaving the shop, but then gets clunky, my bet (as with the others already mentioned) is cable tension.

One question have is whether or not the shop mechanic attempts to shift through as many gears as possible with out the wheel moving. This will properly seat the cables & take up about 90% of the usual, so-called "cable stretch" that is common. From there, the mechanic should then proceed to set the cable indexing on the smallest cog in the usual fashion.

Your shifting issues may simply be the shop you are taking the bike to. Maybe not.

As noted, cadence can mean a lot. Especially with Shimano's lower end groups. The higher end groups tend to not be so flexy or made with such low grade mild stamped steel. The alloys themselves are higher grade & stiffer, even if the part profile & thickness is the same. The higher end groups tend to have more machining to facilitate better shifting. As noted above regardless of part quality level, strain on the chain has an effect on shift quality. It's just more apparent on some groups over others.

It seems that the engineering on the lower end groups went from making an acceptable group at a cost work well to making a group cheaper at the expense of working well. Tourney or Acera of 20 years ago is not the Tourney or Acera of today.

All that being said: If you find that your limit screws, especially the H screw that determines the indexing index just won't stay put, a drop of blue Loctite 242, can make it stay put the way intended. At the very least it won't hurt, especially if the screw has been adjusted a number of times. The cheap nylon lock feature tends to not crasp the screw threads very tightly after a few adjustments. Especially if the metal threads in are poorly/loosly machined as well. Which is usually the case with the worn-out/obsolete manufacturing equipment reserved for the lower-end groups & absence of costly quality control. A drop of Blue Loctite 242 is probably a good idea on the limit screws of any old derailleur that won't hold adjustment well.

But, yeah...If that doesn't work, there's a lot of information in this thread to point you in the right direction.