With GTM, a bot needs to render your pages to see your Structured Data (SD). This means most bots will not see it. Best is to test with any search engine or system you care about.
If you are trying to test with the Structured Data Testing Tool, it may work, it may not. It does not do true rendering. Test with the Rich Results Tester or Google Search Console which does render like Googlebot. I suspect most other testing tools will not pick it up.
Googlebot does render, but Googlers have several times recommended that you avoid using GTM to add structured data. One reason is that a page is not always instantly rendered (they are faster now), so there may be some time where Google is not aware of your SD. It also complicates things, meaning its harder to test and there are more ways to make it fail.
I'd avoid using GTM for SD unless you have a strong need to use it.