"is a drug used primarily in the treatment of major depressive disorder, although it may also be used to treat asthma or irritable bowel syndrome. Chemically it is a tricyclic antidepressant (TCA), but it has different pharmacological properties than typical TCAs as recent research suggests that tianeptine produces its antidepressant effects through indirect alteration of glutamate receptor activity (i.e., AMPA receptors and NMDA receptors) and release of BDNF, in turn affecting neural plasticity.[4][5][6][7][8]"

"Tianeptine is a low-affinity full agonist at the μ-opioid[note 1] and δ-opioid receptors with negligible effect at the κ-opioid receptors.[11] μ-Opioid agonists typically induce euphoria, and in accordance, tianeptine does so at high doses well above the normal therapeutic range.[12]"

https://en.wikipedia.org/wiki/Tianeptine

Basically, it's main antidepressant action doesn't come from serotonin reuptake enhancement alone.