This was reported in my plugin https://github.com/lmscloud-io/moodle-tool_vault/issues/14 and the report confused me initially.

Then I realised that it is a problem with how AMOS imports plugin strings.

In my language file I have

$string['loginexplanation'] = "Create an account on {\$a} to be able to backup or restore the site. With the <b>free account</b> you will be able to backup and restore small sites and store them up to 7 days in the cloud.";

which displays correctly in English. In fact, if I do not use \ in front of '$a' , I get errors

Warning: Undefined variable $a in /.../admin/tool/vault/lang/en/tool_vault.php on line 164

But when my plugin's strings are imported into AMOS they appear in the AMOS UI like this: