We probably never have been so aware of smell and taste as during the past Corona year. There is a post form a medical text [about one-sided loss of smell]

Unilateral anosmia[no able to smell] is very rare and is most often found in local afflictions, such as inflamed nasal mucosa, when they are worse on one side than on the other. It can, however, be an excellent sign for drawing attention to a frontal lobe tumor (Liddell, 1976). It is rare for trauma to affect the olfactory nerve on only one side.

Orient, Jane M.. Sapira’s Art and Science of Bedside Diagnosis . Lippincot (Wolters Kluwer Health). Kindle Edition.