MiniTool Partition Wizard also allows you to extend an existing partition into unallocated space elsewhere on the same physical drive, that's done using the "Move/Resize Partition" option whereas you chose "Merge Partition".
The Windows' built-in Disk Management tool also can extend a partition into adjoining unallocated space but only behind it (which is what you have showing in your pic above). Additionally, if you selected the actual drive (disk 3 in your pic) MiniTool would have been given the additional specific option of "Extend Partition" because the unallocated space is then detected on the selected disk.
Spoiler: Click to expand
Anyway one way or another your problem's been sorted and you solved it yourself too.
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