The reason is simple: both lemon and ACV are acidic. This means that they cause the cuticle layer to contract and flatten. A flattened cuticle layer is closed to the entry of moisture and other products so if you plan on doing a deep condition after the ACV or lemon rinse, you're working against it.
For an effective deep condition you want the cuticle to be more open and to ensure this this you shouldn't be rinsing with something acidic just beforehand.
Once you've done your deep condition and rinsed off the conditioner the ACV or lemon rinse (or whatever other acidic rinse you choose) will close the cuticle layer thereby:
- Boosting hair's shine
- Reducing frizziness
- Making hair less porous so that you retain moisture better and
- It will not work against the deep conditioner
You can further rinse the ACV or lemon rinse with cold water to get any smell off. As the acidic rinse has already done its job of sealing the cuticle layer this follow-on cold water rinse is NOT counter productive.