Failing non-OSGi portlets on Liferay CE 7.4.3.5 GA5 or later

I did some non-OSGi testing with latest Vaadin framework git branch that supports latest Liferay kernel version, 29.0.1.

The portlet was a simple war-file containing the web.xml, portlet.xml, liferay-display.xml, liferay-portlet.xml, liferay-plugin-package.properties and one java-class extending Vaadin's UI class.

The used Vaadin version was a self compiled 8.15-SNAPSHOT from Vaadin framework's github's master branch that contained support for Liferay kernel 29.0.1.

Browser cache/history was cleared between each test.

Liferay versions that passed i.e. the simple portlet appeared as expected:
Liferay CE 7.3.4 GA5
Liferay CE 7.3.5 GA6
Liferay CE 7.3.6 GA7
Liferay CE 7.3.7 GA8
Liferay CE 7.4.0 GA1
Liferay CE 7.4.1 GA2
Liferay CE 7.4.2 GA3
Liferay CE 7.4.3.4 GA4

The two newest Liferay releases failed i.e. the simple portlet did not appear:
Liferay CE 7.4.3.5 GA5 (released 2021-12-28)
Liferay CE 7.4.3.6 GA6 (released 2022-01-06)

There were no errors in browser nor in Liferay log files.

It looked like UI's
protected void init(VaadinRequest request)
was never called.

Setup:
Debian GNU/Linux 10 (buster)
OpenJDK 11
Firefox 78.15.0esr
Standard Liferay Docker images

Reproduce the non-OSGi WAR portlet with the listed web.xml, portlet.xml, liferay-display.xml, liferay-portlet.xml, and liferay-plugin-package.properties files on Liferay CE 7.4.3.5 and 7.4.3.6. Start at the Vaadin UI entry point, protected void init(VaadinRequest request), and compare the working and failing Liferay versions. Done means the portlet appears and init is invoked on the affected releases, with the compatibility cause covered by a regression test if the project supports one.