We have just upgraded to Moodle 3.5.3 (Build: 20181112) and are using the Boost theme. We are also experiencing the same issue - if the glossary is set to 'Simple Dictionary Style' the glossary Actions Menu (cog) is missing. I have noticed that in the breadcrumbs if it says "Browse by Alphabet" or "Browse by Category" the cog does not display. I did manage to somehow remove this from the breadcrumbs and the cog displayed but I am unable to repeat this.
The easiest work around I found was to change the Appearance - Display Format to something other than 'Simple Dictionary Style', the cog would appear so then I could use the cog to export / import my entries. Then edit the settings and change my display format back to 'Simple Dictionary Style. Thanks