Unfortunately this is by design. MSFS now displays a random tail number based on the format used by the country of the departure airport. Although it’s definitely a bug that entering the tail number when selecting an aircraft is ignored.
Another bug is that MSFS will now always display a dynamic tail number on any aircraft that has a dynamic tail number. Any livery that needs to match a specific tail number font/location needs to disable the dynamic tail number, to prevent 2 tail numbers appearing. Many airline liveries need a fixed tail number to accurately replicate the livery.
Unfortunately the poorly implemented new dynamic tail number process also stops methods used to hide or disable the dynamic tail number. So what works in 2020 doesn’t work in 2024. This effectively blocks a whole swag of custom liveries from ever being used in 2024.
And the new dynamic tail number has one last casualty. In the past livery creators could recolour the dynamic tail number to match the livery by editing the panel.cfg. This also no longer works in 2024, rendering another group of liveries unsuitable for use in 2024. Based on information in the SDK it looks like aircraft designers may be able to fix this problem though.
I think a quote from Jurassic Park is pertinent here in relation to the success of the new dynamic tail number. “They were so preoccupied with whether they could, they didn’t stop to think if they should”.