That is somewhat a consequence of grip issue:
If you do a FH oriented grip change (like Timo and Ochtarov do) it naturally closes the angle to 45 degrees on backswing and closes it further at the end of the stroke.
Most Chinese players also switch grips (watch in slow motion during rallies: ) but tend to stick to neutral grip for FH and switch back to BH grip for most other shots.
The reason the grip switch is needed is because the wrist flexion and extension is natural for Fh grip while doing FH and Bh grip for BH. Instead, if you keep the same BH grip for doing FHs, the wrist turns into radial and ulnar deviation which does allow you spin the ball but you lose speed because the latter radial/ulnar deviation is much slower than flexion/extension. You can of course force a close angle even with BH grip during FH but at that point you introduce muscle tension which again slows down the shot more.
My coach suggested I atleast switch to neutral grip for FH (like Ti Long talk about here at 4:30 min mark:
)