To confirm, you have a late production 544 with the B18 and M40?
This is a diagram of the shift lever arrangement on the early M40 / M41 on the 140. Should be the same as a late 544
https://www.skandix.de/en/installati...60265/2000715/
On my M41 (and on the limited number of other M40 / M41s that I have seen) the threaded opening on the top of the transmission has slots cut into it on the left and right side of the opening. Pins on the shift lever drop into the slots and should prevent the the lever from rotating (it can rock; but, should not rotate). However, that diagram from Skandix does not show the pins or the slots in the top opening which leaves me wondering if there is a different / older version of the top cover and shift lever.
On my style of shift lever, the slots in the shift lever opening and the pins on the shift lever itself can wear leading to a lot of slop / play in the shift lever. The fix for this is to grind out the slots a bit and then fit bushes to the pins so that the pins are a tight fit in the cover. On the Swedespeed forum I remember a thread from perhaps 5 - 6+ years ago describing this process.
If there is a shift lever that does not have those locating pins I don't know what mechanism restrains the lever from rotating. I suspect that you will have to pull back the shift boot. This should allow you to access the top of the transmission cover and unscrew the retaining collar (big nut that arcturus refers to) and see what is going on. As shown in the diagram, there is a ball socket 'down there' that can wear and a spring; but, I don't remember it being adjustable.
Also be aware that the shift lever is two piece, inner and outer with the outer attached to the inner by two rubber bushings. These bushings wear and can allow the outer lever to rotate on the inner lever and add a whole lot of flex to the the shift lever; but, they would not allow the shift lever to rotate on the top of the transmission cover.
You have a new and interesting problem. Report back on what you find.