When writing a command, the game displays a list of suggestions about what you may want to write next (unless you disabled that). But if the suggestions appear under the cursor, it won't need to be moved to be active, hence selecting selecting by default the suggestion the mouse is on. Which can be annoying as pressing Tab will select that suggestion instead of the first one (and Tab is most often used to complete what is usually already on the first or second line of the suggestion list.
Of course, in practice, it has more chances to happen for commands with long lists of suggestions and long IDs, such as blocks (/give, /fill...) and scoreboard criterias. It may happen more easily on smaller screens, too (was on 1366×768 before, now 1920×1080, barely used commands for but I feel like it may happen less easily for me now. I may be wrong though, too early to tell).
• Copy-paste the following : /give @p minecraft:jun
• Put your mouse above the suggestions that appear (preferably slightly above the text bar. As long as it's not on the first suggestion, it's okay). Just so that you know it's where you need it to be to reproduce the bug. Delete the command (without closing the chat, or you will have to place the mouse again), then paste it again, or type it if you prefer.
• As you can see, the suggestion your mouse is on is highlighted. Type "g" if you pasted and want to make sure it won't change anything.
• Press Tab, it will enter in chat bar the suggestion the mouse was on, instead of the first suggestion.