Yes players lose development as they gain stars, and will still lose development even if those said skill points obtained are not applied.
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I had a 18 yo MC last season I've mentioned him a few times in other topics. he was a few SP from scout.
I allocated the SP as they were awarded. :(
As a five star he trained at around on Avg at 140 progress per day after allocation and gaining his 6* that dropped to 111 these figures were obtained over a 10 day period at the end of last season.
This season back to 5* L6 he now trains at 163 progress again figure calculated since day 1 up to day 8
Match/Games P8 + 1sub gains 957
Practice games P5 gains 254
Cardio 2 sessions 61
Stretching 2 sessions 33
He missed one day of training through injury.
As others have suggested and IMO this goes a long way to proving the theories, do not add any SP that will improve * rating.
The above increase also apply to older players pro rata to their own previous gains.
I'm also finding up to 30-50% variants in training gains depending on age and when they actualy under take training.
Training in the morning before any games yeilds the 30-50 drop in progress, highest gains are after a match and especially so if the player has been given MOM where I'm finding up to 25% gains over the Avg.
All the above is on Normal intensity.
Maybe it's the continuous training that causes the drop as explained above, data doesn't lie, and yes it has been checked and rechecked for errors.
Just done a few sessions on a 22yo 4* 43 striker (kissi) no other training or games before the session he had 97% condition at the start, still no drop in morale at this point.
Practice 111 (3)
Cardio 1111 (4)
Stretch 11111 (5)
Gains
Practice 120/65/67
Cardio 52/49/41/45
Stretch 23/19/17/28/23
SP 5
Rest packs
111111