Open https://bugs.documentfoundation.org/attachment.cgi?id=168523 and link the database to LO through Tools → Options → LibreOffice Base. Open the *.odt-file. Edit the form. Right click on a field and chose the properties for the form. Seems data source is empty. Chose the data source "Beispiel_Sport_HSQLDB". Chose the table "rel_Teilnehmer_Sportart". Save the form. Close the form. Open the form again. Edit the form and have a look: The data source has been gone. This behavior appears with LO 7.1.0.1 and OpenSUSE 15.2. 64bit rpm Linux. It doesn't happen with LO 7.0.4.2.
I tried on Ubuntu 20.04 using bibisect-linux-64-7.1 and the data source was not empty. The form worked, and editing the form showed the table information already there in the form properties data tab.
Steps to reproduce are simply Step 0) link the database to LO through Tools → Options → LibreOffice Base. 1.) Open the *.odt-file. The combo-boxes should have something inside. Bibisect-linux-64-7.1 points to author Noel Grandin on 2020-11-11 06:33:53 with commit 3de38e95561ab7ca114d9f3307702ba89c4e3e9a use fastparser in forms
This seems to have been fixed in 7.1 by author Michael Stahl on 2021-01-11 10:29:48 with commit db283d2e3f5cd8be412c5204f358cd06af761b13 xmloff: "form:datasource" vs. "db:data-source" Recently xmloff started to use XML_DATASOURCE for "form:datasource" but it actually means "data-source". Add XML_DATA_SOURCE and use it for "db:data-source". (regression from 3de38e95561ab7ca114d9f3307702ba89c4e3e9a)