Why Some Oak Aged Wines Can Taste Bitter

I saw this article in one of my wine feeds which appeared in Wine Spectator. It was called French Researchers Unlock a Secret to Wine Bitterness: Oak Barrels. This article peaked my interest so I decided to read the original article.
Historically, oak barrels have been used in the transport of wines. Now, oak barrels are used in the fermentation and/or aging of wine. It is well known that this contact can modify the sensory properties of the wine. Compounds in the oak are responsible for changes in color, aromas, tactile sensations and taste. For example, a gain in sweetness is frequently observed during aging and can be explained by the release of sweet triterpenoids from oak wood.
Occasionally, however, aging in barrels can sometimes increase the perception of bitterness. This bitterness was initially attributed to ellagitannins, but their low concentrations suggested their limited influence on wine bitterness.
The authors of this research set out to identify the compounds presumed to be responsible for the sensation of bitterness in white and red wines.
Oak wood contains (+)-lyoniresinol which is known to be very bitter as well as coumarins, which are secondary metabolites originating from the phenyl-propanoid pathway via t-cinnamic acid.
Coumarin is well known for its characteristic odor reminiscent of vanilla pod but also of almond, together with the scent of cut hay. The authors focussed their research on coumarin and coumarin derivatives. They noted the presence of esculetin, fraxetin, umbelliferone, and coumarin in the oaked red wine, whereas scopoletin and 4-methylumbelliferone were not detected in red wine.¹

Tasting evaluation of the six compounds, esculetin, fraxetin, umbelliferone, coumarin, scopoletin and 4-methylumbelliferone showed that five of the six targeted coumarins were described as bitter. 4-methylumbelliferone was the most intense, followed by esculetin, scopoletin, then coumarin and umbelliferone.
In addition, 90 commercial wines, 44 red wines (24 from Bordeaux, 6 from Burgundy, 2 from Languedoc-Roussillon, 2 from Rhône Valley, 1 from Loire Valley, 1 from Beaujolais, 7 from Spain, and 1 from Germany) and 46 white wines (13 from Burgundy, 11 from Alsace, 9 from Bordeaux, 5 from Loire Valley, 4 from Languedoc-Roussillon, and 4 from Rhône Valley) with vintages from 1995 to 2017 were assessed for the coumarin concentration. Using the LC-HRMS method previously validated, different concentrations of the various coumarins were observed depending on the sample. In general, higher concentrations of coumarins were observed in red wines, possibly because more new barrels are used in the aging of red wines.
Apart from their presence, the sensory contribution of coumarins to the taste of wines and spirits remained largely unclear.

References:
1. Delphine Winstel, Eric Gautier, and Axel Marchal, Role of Oak Coumarins in the Taste of Wines and Spirits: Identification, Quantitation, and Sensory Contribution through Perceptive Interactions,J. Agric. Food Chem., 2020, 68, 7434−7443.