As per comment below, I have confirmed this same issue occurs using debian-live-10.0.0-amd64-cinnamon.iso -- suspect there is some kind of GTK issue about touch-input, and the Cinnamon menu is just a quick way of triggering the issue.
NB: Seems to help to open text editor full-screen behind, highlight text there using touch first, THEN use multitouch digitizer to open mint-menu, this will almost certainly give you the case you can't select on Cinnamon lm-menu using touchscreen.
In IRC, Acheron // Linux-Advocate confirmed this using a HP Pavilion x360 Convertible , I have Asus X202E ...
NB: I'm coming to conclusion this is not a new issue and also is something in GTK/wider that affects pop-up applets and menus etc. too. See also MATE ISO image comments.
Been testing this 19.2 Cinnamon BETA 64bit on Touchscreen machines [I have more to say about rotation and multi screens etc], BUT -- I have one, simple, reproducible bug:-.
(1) boot iso
(2) Press Mint menu with touchsrceen, can correctly select office -> LibreOffice Writer -> etc [close this]
(3) Open Firefox, leave this open fullscreen (shows some error text as not connected to network)
(4) Press Mint menu on touchscreen again, this time CAN'T higlight Office [won't go green], only moves mouse-cursor! , but pressing firmly and moving, shows that you are infact focussed/selecting text on the firefox window behind! When opened via touchscreen like this, even touchpad-mouse cannot select Office.
(5) Whereas, using touchpad it IS possible to select Mint menu with proper focus. One opened like that, using touchscreen can select Office -> Libreoffice Writer etc.
Subtle, but very annoying and reliably-reproducible bug on this Asus i3-netbook 1366x768 screen and Atmel maXTouch Digitizer integrated.
This ISO features the latest updates for ubiquity, cinnamon and the themes.