I have tab containing a single group field which contains
• several Query Drop Down (Single Select) fields
• several Text fields
• several Drop Down (Single Select) field

The tab is displayed depending on whether another field, external to the field group, is set to Yes or No.

The second of the Query Drop Down (Single Select) fields is (probably unnecessarily) displayed only if the external field is set to No.

One of the Text fields is displayed only for a particular value in the second of the conditional Query Drop Down (Single Select) fields.

As this conditional text field was not updating on the database I updated my test installation with the latest CB Conditional to see if that would fix the problem. It did, but now three of the Query Drop Down (Single Select) fields in the group are not being saved to the database and are being reset to empty. This data is not saved both on entering new data and on updating the fields.

I have tried removing the condition from the second of the Query Drop Down (Single Select) fields but it made no difference to the outcome.
I have very carefully checked and compared the settings for all the fields in the group (including all the Integrations) to ensure that they are the same on both the live system and the test system. The only difference I have detected is that in the latest version of CB Conditional there is now a User field (Displayed/Viewing). Otherwise the only difference between the two installations is the version of CB Conditional.

In both installations the CB Conditional parameters values are:
Backend: Enable
Reset: Enable
Debug: Disable

Live versions
PHP: 7.3.27
Server version: 10.3.28-MariaDB
CB Conditional: 5.0.0+build.2020.08.12.15.46.15.ae35739f0

Test (localhost) versions
PHP: 7.4.15
Server version: 10.4.17-MariaDB
CB Conditional: 6.0.0+build.2021.03.23.15.23.53.94e7f2fd2

Both installations
Joomla!: 3.9.25
CB: 2.5.0+build.2021.04.09.20.27.16.19d16d5eb

I cannot bring my live installation up-to-date until this problem is resolved otherwise I may lose a lot of data.

Thank you.

I have set up a new copy of my live site this morning, so I will now update it with absolutely everything so that I have a totally up-to-date version and try again.

Is it possible to get a second site key so that I can 'install from web' on my localhost or is that an ask too far?

Excellent - thank you

I have updated everything, including plugins that are installed not not published (because we don't use them).

Three of the Query Drop Down (Single Select) fields are still not being saved but have been given NULL values. I checked the actual values in comprofiler.

I unpublished CB Conditional as you suggested, and on re-entering the edit the values were saved correctly. Again, I checked the actual values in comprofiler.

Just in case it's relevant, these three fields were previously set to Field Conditional Hide but all were reset to CB Normal Settings when the condition was no longer needed.