Oh......too much information 
I think I can confirm the problem Using Gimp 3.0.6 (appimage)
(1) The sub-image with layer mask / text with layer effect (fx) / saved as a Gimp .xcf
(2) Opened as a layer into a base image. This loses the layer mask. It is the same if the sub-image is drag-dropped or copied-pasted.
(3) The fx applied to the text. Text properties are lost but the layer mask is retained. Just a reminder that with Gimp 3.0.6 text fill and outline is now fixed in the text tool options.
What to do ? It does look like a bug. There is one Gimp dev who sometimes visits otherwise post a (short) bug report to:
https://gitlab.gnome.org/GNOME/gimp/-/issues
I think I can confirm the problem Using Gimp 3.0.6 (appimage)
(1) The sub-image with layer mask / text with layer effect (fx) / saved as a Gimp .xcf
(2) Opened as a layer into a base image. This loses the layer mask. It is the same if the sub-image is drag-dropped or copied-pasted.
(3) The fx applied to the text. Text properties are lost but the layer mask is retained. Just a reminder that with Gimp 3.0.6 text fill and outline is now fixed in the text tool options.
What to do ? It does look like a bug. There is one Gimp dev who sometimes visits otherwise post a (short) bug report to:
https://gitlab.gnome.org/GNOME/gimp/-/issues