As first the relevant section of the console logs:
[B#693] 2021/09/19 20:52:55 [SEVERE] java.lang.NullPointerException
[B#693] 2021/09/19 20:52:55 [SEVERE] at net.technicpack.utilslib.Memory.getClosestAvailableMemory(Memory.java:73)
[B#693] 2021/09/19 20:52:55 [SEVERE] at net.technicpack.launchercore.modpacks.RunData.getValidMemory(RunData.java:84)
[B#693] 2021/09/19 20:52:55 [SEVERE] at net.technicpack.launcher.ui.components.FixRunDataDialog.recommendSettings(FixRunDataDialog.java:96)
[B#693] 2021/09/19 20:52:55 [SEVERE] at net.technicpack.launcher.ui.components.FixRunDataDialog.<init>(FixRunDataDialog.java:52)
[B#693] 2021/09/19 20:52:55 [SEVERE] at net.technicpack.launcher.launch.Installer$1.run(Installer.java:145)
[B#693] 2021/09/19 20:52:55 [SEVERE] at java.base/java.lang.Thread.run(Thread.java:829)
[B#693] 2021/09/19 20:54:47 [INFO] Attempting to open /home/***/.technic/logs
[B#693] 2021/09/19 20:54:47 [INFO] Using file:/home/***/.technic/logs/
[B#693] 2021/09/19 20:54:47 [INFO] Intermediary path file:///home/***/.technic/logs/
This only happens, when the modpack selected specifies a minimum memory requirement, which is not fulfilled. Normally a window detailing this problem would show up, but I guess there is the problem. This bug does not happen when the memory requirement is satisfied and appears on all platforms (Windows, Linux, Mac).
The launcher does not hide or react in any other way than displaying the "Play" button after pressing.