Corruption on the bootstrapper progress dialog when installing from the options page

[Numpsy]

Hi,
I'm a beginner with Wix and am not sure if this is officially supported behaviour, and also not sure if it's related to https://github.com/orgs/wixtoolset/discussions/7373, but:

I've inherited an installer that uses Wix 3.10, and have been having a go at updating it to v4.
I've got a pair of msis working ok, but there is some graphical corruption on the progress page of the installer. The situation is this

The bootstrapper is using a custom version of the rtf theme which has

1. A set of radio buttons on the Install page
2. Has the EULA and install button on the Options page

That part displays ok, but then when I run the install, the progress bar and such get rendered over the top of the EULA field rather than getiting a clean display.

As a test to try and rule out some incorrectness in the old modified theme, I tried doing something similar with a newly created, otherwise unchanged project.
I found that if I change the Options page on the hyperlinklicense theme to

        <Page Name="Options">
            <Label X="11" Y="80" Width="-11" Height="30" FontId="2" DisablePrefix="yes">#(loc.OptionsHeader)</Label>
            <Label X="11" Y="121" Width="-11" Height="17" FontId="3" DisablePrefix="yes">#(loc.OptionsLocationLabel)</Label>
            <Editbox Name="InstallFolder" X="11" Y="143" Width="-91" Height="21" TabStop="yes" FontId="3" FileSystemAutoComplete="yes" Visible="no" />
            <Button Name="BrowseButton" X="-11" Y="142" Width="75" Height="23" TabStop="yes" FontId="3">
                <Text>#(loc.OptionsBrowseButton)</Text>
                <BrowseDirectoryAction VariableName="InstallFolder" />
            </Button>
            <Button Name="OptionsOkButton" X="-171" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">
                <Text>#(loc.OptionsOkButton)</Text>
                <ChangePageAction Page="Install" />
            </Button>
            <Button Name="InstallButton" X="-91" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">#(loc.InstallInstallButton)</Button>
            <Button Name="OptionsCancelButton" X="-11" Y="-11" Width="75" Height="23" TabStop="yes" FontId="0">
                <Text>#(loc.OptionsCancelButton)</Text>
                <ChangePageAction Page="Install" Cancel="yes" />
            </Button>
        </Page>

Then the options page displays like this, which is ok:

And clicking the install button starts the install, but then this happens:

Where the progress bar gets shown on top of the folder selection fields, which isn't very useful.