TimerEvent behavior is wrong on STM32 Mbed boards

I made two program for checking TimerEvent.
Run those two program on three boards, Nucleo-F446RE, -L152RE and FRDM-K64.
Result is below.


Newest revision OS5.15.3, 1st timing is not expected value 50us.
Old revision OS2.132, all of results are okay(some overhead time).

You can see this forum question.

Famous in Mbed world, SoftSerial by Erik is not run correctly on Nucleo-L432KC.
SoftSerial is using this TimerEvent function and 1st event is very important for both TX and RX.
As a result, we cannot use SoftSerial library on Mbed STM32 series with latest revision.
I don’t know real problem(low level implementation of a timer related area?) and cannot solve by myself.
Please solve this issue.