I wonder it was working before. To me, it looks like a side-effect of RequireJS wanting to register it as a module. For scripts outside the RequireJS toolchain in Plone you should normally register them as "legacy bundle" - that is setting the "compile" flag of the bundle to false. Then the bundle is wrapped within a de- and re-registration of requirejs, like here: https://github.com/plone/Products.CMFPlone/blob/009f785e450430ee7b143624480aef9268491c0b/Products/CMFPlone/resources/browser/scripts.pt#L6
Try:
<script>
var _old_define = define;
var _old_require = require;
define = undefined;
require = undefined;
</script>
<script src="YOURSCRIPT"/>
<script>
define = _old_define;
require = _old_require;
</script>
I myself know many swearwords about this. This will change in Plone 6 by getting rid of RequireJS and simplifying the resource registry.