Since, currently, fermatas don’t hold (noticeably) in playback, I inserted some tempi changes at their locations followed by another to bring the tempo back, effectively causing a pause.
I then hid the markings i.e. metronome mark and text not shown in the properties panel and they all vanished beautifully.
Then I wanted to change the tempo of one of them but they had all vanished so completely that I couldn’t find them, neither by selecting whole bars nor through using Edit->Filter->All tempos.
Is there another way to find them again?