When an old syntax tvar (<tvar|var>) is used in a template, it breaks the template because the | is seen as a new argument separator.
As an example:
$ echo '{{1x|<tvar|1d>test</> test}}' | php bin/parse.php --wt2html --scrubWikitext <p data-parsoid='{"dsr":[0,28,0,0]}'><span about="#mwt1" typeof="mw:Transclusion" data-parsoid='{"pi":[[{"k":"1"}]],"dsr":[0,28,null,null]}' data-mw='{"parts":[{"template":{"target":{"wt":"1x","href":"./Template:1x"},"params":{"1":{"wt":"<tvar|1d>test</> test"}},"i":0}}]}'><tvar</span></p>
We'd expect the "test" content to be visible akin to this:
$ echo '{{1x|<tvar>test</tvar> test}}' | php bin/parse.php --wt2html <p data-parsoid='{"dsr":[0,29,0,0]}'><span about="#mwt1" typeof="mw:Transclusion" data-parsoid='{"pi":[[{"k":"1"}]],"dsr":[0,29,null,null]}' data-mw='{"parts":[{"template":{"target":{"wt":"1x","href":"./Template:1x"},"params":{"1":{"wt":"<tvar>test</tvar> test"}},"i":0}}]}'>test test</span></p>