It's CRL http url does not have a trailing slash. RouterOS refuses to load information from such URL. Moreover, Webfig claims that URLs without a trailing slash are not valid.
Compare dynamic CRL and manual. Manual works OK. For dynamic CRL logs show:
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Jun/02/2018 17:59:55 certificate,debug flush CRLs
Jun/02/2018 17:59:55 certificate,debug start CRL update
Jun/02/2018 17:59:55 certificate,debug bad CRL URL: http://crl.root-x1.letsencrypt.org
Jun/02/2018 17:59:55 certificate,debug CRL fetch failed: