FileVaultを無効にして改善しないなら、Appleの対応待ちでしょう。

Q: I connected my monitor to the Breakaway Box or Puck on my 2018 or newer Mac mini, but it boots with a black screen.

A: If you have a 2018 or newer Mac mini with FileVault turned on, you must connect your primary display directly to the HDMI port on your Mac mini during startup
in order to enter your encryption password. After you log in and see the macOS Desktop, you can unplug the display from Mac mini and connect it to your eGPU.

Under macOS 10.14.6+, a 2018 or newer Mac mini with some GPU cards must be booted with a monitor connected to the Mac mini's HDMI port.
If you have a Mac mini with an affected GPU card, you must either:
(a) Connect an HDMI display emulator plug to the Mac mini HDMI port and mirror displays.
(You must make sure that the HDMI display emulator is the Master Mirror. Drag the menu bar to the HDMI display emulator window before mirroring.)
---or---
(b) Connect a monitor to the Mac mini's HDMI port to log in, and then move the HDMI monitor connection to the eGPU.

http://www.sonnettech.com/support/kb/kb.php?cat=524&;expand=_a2_b914_b1084&action=b1110#b1110