Here is a little flow to test it. Select option "abc" and it sends the string "abc", select option 1.0 and it sends number 1 instead of string "1.0". I think that has to be a bug doesn't it?
[{"id":"65a8817f.1c618","type":"ui_dropdown","z":"bdd7be38.d3b55","name":"","label":"","tooltip":"","place":"Select option","group":"a8e4a3.d31edb6","order":3,"width":0,"height":0,"passthru":true,"multiple":false,"options":[{"label":"","value":"1.0","type":"str"},{"label":"","value":"abc","type":"str"}],"payload":"","topic":"","x":210,"y":4760,"wires":[["8f66b34d.841898"]]},{"id":"8f66b34d.841898","type":"debug","z":"bdd7be38.d3b55","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":420,"y":4780,"wires":[]},{"id":"a8e4a3.d31edb6","type":"ui_group","name":"Chart","tab":"8032fe72.980b98","order":3,"disp":false,"width":"6","collapse":false},{"id":"8032fe72.980b98","type":"ui_tab","name":"PnH","icon":"dashboard","order":1,"disabled":false,"hidden":false}]
I have reverted to 2.19.4 and confirmed that it works correctly there, I have also confirmed that if a dropdown is added in 2.19.4 and the dashboard updated that Allow Multiple becomes checked, which I think is another bug as it changes the way the node works.