After more testing Evo's Maia1900 ELO rating has still been fluctuating a fair amount - in the region of 100 points - and now after 36 games has held a rating of 2208 for two consecutive games. It needs more test games before I can possibly achieve a more definitive estimate using Shredder's ELO estimation method. Assuming Shredder's calculations are correct then all I would say at the moment is that Maia1900 with 6ply search has a rating probably at least 2100.
I have noticed that Maia does lack certain end game knowledge such as King and Rook against King as mentioned before. However, I came across the following position with Maia playing Black and Black to move and it appears this confuses the Maia engine as it seems to get stuck!
The game is a draw as Black must take the Rook with his Bishop, but I left Maia running for 10 minutes and no move was given. This is way longer than is required to search 6 ply on the Evo. I then tried the built-in Lc0 engine with it's default weight file and this was the same, so there is a bug in the Evo's Lc0 engine. I suspected this might be the case because Maia uses the Lc0 engine and Maia is really just a set of weight files used by Lc0.
Ray