As far as I can tell the problem is that Joomla is trying to get the client information by name but the update XML needs to define it by numeric ID (at least if we want the update XML file to also work on Joomla 3).

Please also note that the official documentation also says the same:

client – Required for modules and templates as of 3.2.0. – The client ID of the extension, which can be found by looking inside the #_extensions table. To date, use 0 for "site" and 1 for "administrator". Warning! Plugins and front-end modules are automatically installed with a client of 0 (site), but you will need to specify the client in an update or it will default to 1 (administrator) and then found update would not be shown because it would not match any extension. Components are automatically installed with a client of 1, which is currently the default.

Warning: The tag name is for Joomla! 2.5 and <client_id> for 1.6 and 1.7. If you use <client_id> (rather than ) on a 2.5 site, it will be ignored.

So, this is a case of Joomla not following its own documentation.

The correct fix is to check if $this->latest->client is numeric (AS IT SHOULD, PER THE DOCUMENTATION AND BECAUSE THIS IS HOW JOOMLA 3 WORKS!). If it's numeric, assign $this->latest->client_id = $this->latest->client. Otherwise go through Application helper.

To make it clear, the offending line should read:

$this->latest->client_id = is_numeric($this->latest->client) ? $this->latest->client : ApplicationHelper::getClientInfo($this->latest->client, true)->id; 

DO NOT change the documentation to make a string again. This will break updates for extensions which are compatible with both Joomla 3 and 4. We already had this b/c break BS going from Joomla 1.7 to 2.5. We threw Joomla 1.6 and 1.7 users under the bus back then which was acceptable because 1.6 and 1.7 were STS and immediately EOL'ed when 2.5 came out. This is not an option with Joomla 3. If we have to choose between supporting our Joomla 3 or our Joomla 4 clients we will all support Joomla 3 clients and tell them to NOT upgrade to Joomla 4 before August 17th, 2023.

Recently reported again for other 3rd party extensions: #35815 .

If the project is actually interested in solving this I can make the one line PR for the change I have already commented. I need either @wilsonge or @HLeithner to give me to go–ahead. Until this is fixed this problem will keep happening. It's not a matter of “whether”, it's just a matter of “how often”.

@nikosdion Will it also solve #29778 ? Or is that a different thing?

@richard67 Correct. It is the exact same issue.

@nikosdion yes please go ahead and support the numeric id, can you may add a note to deprecated this for 5.0? or at least a "reconsider" comment?

Okey-dokey! On it.

Pull request created: #35822

@nikosdion Could you mention the other issue #29778 too in the description of your PR? Thanks in advance.

Closing as having a pull request. Please test #35822 . Thanks in advance.

Done!