Hello,
If a default installation was done, then nothing should have changed ideally.
When you start command _HATCH in AutoCAD Mechanical 2018 (and you don't have ribbons disabled) you should get the full hatch ribbon display.
One way to disable the ribbon switching to the hatch-creation tab would be to modify the CUI.
Start command _CUI, on the left side in the tree go to AMPP ==> Ribbon ==> Contextual Tab States ==> Hatch Creation
Now how is the "Contextual Display Type" set on the right side?
If it's not set to "Full with focus" ... then change it to "Full with focus"

If the Contextual Display type is showing blank, that means that there were some customizations done (or maybe migration and that failed).
If you do not have any other customizations, and still cannot get the Hatch Contextual tab,
Start again command _CUI, and then right-click the ACAD on the top of the left tree ==> try first "Restore ACADM.CUIX", if that does not help then run "Reset ACADM.CUIX"
Hope this helps,
Please mark Accept as Solution if your question is answered. Kudos gladly accepted.
Thank You,
Gaurav Satalkar
QA Engineer