[Bug] with two mod tap keys and RETRO_TAPPING, the first tap is ignored

I am unsure if this is a bug or expected behaviour. I've managed to simulate the behaviour I wanted with PERMISSIVE_HOLD and fine-tuning TAPPING_TERM to my preference, but nevertheless this surprised me.

Replicating:

• in config, RETRO_TAPPING activated and TAPPING_TERM set to something low (or zero)
• layout with two mod tap keys, eg LCTL_T(KC_MINS) and LSFT_T(KC_TAB)
• hold one mod tap key, eg. LCTL_T
• tap the other key, eg LSFT_T

What I expected: ctrl+tab is sent.

What happens: nothing is sent on the first tap. From the second tap onwards, the expected behaviour happens. So for whatever reason the first tap is eaten up.

System Information

• Keyboard: jj40
• Operating system: linux
• AVR GCC version: 10.2.0
• ARM GCC version: 10.2.0
• QMK Firmware version: 0.11.23
• Any keyboard related software installed? No, just xkb.

This is the configuration I use: https://github.com/vaaas/dotfiles/tree/master/qmk/jjvas