Mixamo's source data definitely doesn't follow same conventions, bone structure, naming, rig etc ... it generates issues ... here it looks like the data was automatically processed to keep feet contact with a floor without care of specific kind of motion.
Motion is baked into bone rotations keyframes per frame, that make it quite difficult even for just 30 frames.
Luckily for you Adobe released a Control-Rig addon that make this fix quite simple, because addon generates also IK controllers, that let you animate a leg almost by one bone.
In your specific situation you would have to fix only Z axis of this controller by just 2 or 3 keyframes.
One of the well knows issue is wrong interpretation of bone orientation. Import FBX addon has a feature Automatic Bone Orientation, but it will mess Mixamo's addon as well. Instead ... upload your character (or download from Mixamo and re-upload "as your model") to let Mixamo run through its auto-rig process, assign motion and download as FBX. Import FBX (without any changes in import properties) into Blender.
Download, install, enable addon ...
and if all worked as supposed to, just press Create Control Rig in 3Dview > Mixamo side (N) panel.
Now when feet use IK rig, all you need to do is correct damaged Z information.
Right Feet (fix from frame 5)
Left Feet (fix from frame 17)
For the left foot you would have to fix also Foot Rotation ...
(copy first keyframes and move them on X axis to frame 30 and one more keyframe to replicate motion from frames 1-15 ... or just delete 16-30, duplicate 1-15 and Mirror on X axis)
Note: I used a screenshot of sideview (3) on frame 1 to get a pose reference and drag&droped into viewport to match on frame 30 (not 29). Since all bones are OK up to Hip bone you can nicely see if you match a loop animation.
For more informations about addon or usage of Mixamo data in Blender check this video.