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
Thanks for your post which prompted me to investigate things further. I discovered by chance that you can get the cog back simply by clicking on the link for the glossary name ('Glossary' in your screenshot).
Thus, it seemed that the cog only disappeared when you tried browsing by category or using the index etc.
I tried with several display formats (not just simple, dictionary style) and each one had the same problem - as soon as you try browsing by category etc. the cog disappears.
You're welcome to try it out on our school demo site here: https://school.demo.moodle.net/mod/glossary/view.php?id=650 (log in as a teacher).
I created a bug report - MDL-64856 - for developers to investigate and fix.