STEPS TO REPRODUCE
1. Select several files
2. Type 'F2'
3. Use a rename expression such as: [date:"yyyy-MM-dd_HH-mm-ss"]_[file]{lower}{range:16,}.[ext]{lower}

OBSERVED RESULT

Only the first modifier (the first {lower} here) is now taken into account
{range:16,} and the second {lower} modifier are ignored (and they are with a red background in the expression)
Removing the first modifier (the first {lower}) makes the next one working ({range:16,} here)
The first (and so working) modifier do not have the red background

EXPECTED RESULT

All modifiers should be taken into account.
This is a regression. It has been working for several years (at least since 2010)

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Debian unstable / kde 5.90.0
Qt Version: 5.15.2