Can't use custom markers when opening a file from Inkscape

Steps to reproduce:

• open Inkscape
• open no_custom_markers.svg
• draw any object, convert to a marker
• try to use the marker (or one of the other custom markers that exist in the defs section) on the straight line

What happened?

The new marker is not available in the marker drop-down list in the Fill & Stroke dialog.

What should have happened?

New markers should always appear in the marker dropdown list.

It works as expected in a new, empty file.

Inkscape Version and Operating System:

• Inkscape Version: 1.0beta2 (998e94ed, 2020-02-11)
• Operating System: Linux Mint 19.3