gg5, koj

Even when the word contains non-joyo kanji, we typically hide uncommon 混ぜ書き forms.

@@ -8 +8,2 @@
-<keb>憤まん</keb>
+<keb>忿懣</keb>
+<ke_inf>&rK;</ke_inf>
@@ -11,2 +12,2 @@
-<keb>忿懣</keb>
-<ke_inf>&rK;</ke_inf>
+<keb>憤まん</keb>
+<ke_inf>&sK;</ke_inf>
@@ -23 +24,2 @@
-<gloss>anger</gloss>
+<gloss>(pent-up) anger</gloss>
+<gloss>indignation</gloss>
@@ -25 +26,0 @@
-<gloss>indignation</gloss>

憤懣	25636	86.3%
憤まん	1593	5.4%
ふんまん	1992	6.7%
忿懣	486	1.6%
忿まん	0	0.0%

https://ejje.weblio.jp/content/憤懣
JST科学技術用語日英対訳辞書での「憤懣」の英訳
resentment

クロスランゲージ 37分野専門語辞書での「憤懣」の英訳
resentment

斎藤和英大辞典での「憤懣」の英訳
名詞Indignation; resentment; vexation

Probably shouldn't [sK] 憤まん. Common enough, and this is the "official" form without the non-joyo/non-jinmeiyo kanji.  41 actual results on mainichi.jp(google):
site:mainichi.jp "憤まん"
(vs. 18 actual pages with the full kanji form, always with furigana)

Outside of the expression "much to my chagrin", I think that word is [dated] in English for this sense, or at lest evokes a rather weak level of annoyance (like "awww shucks") compared to this term.

@@ -7,0 +8,3 @@
+<keb>憤まん</keb>
+</k_ele>
+<k_ele>
@@ -10,4 +12,0 @@
-</k_ele>
-<k_ele>
-<keb>憤まん</keb>
-<ke_inf>&sK;</ke_inf>
@@ -27 +25,0 @@
-<gloss>chagrin</gloss>

Smaller kokugos don't have [vs]. I only see it in daijirin.

Google N-gram Corpus Counts
╭─ーーーーー─┬────────┬───────╮
│ 憤懣　　　 │ 25,636 │ 99.4% │
│ 憤懣し　　 │    105 │  0.4% │
│ 憤懣する　 │     37 │  0.1% │
│ 憤懣できる │      0 │  0.0% │
│ 憤懣され　 │      0 │  0.0% │
╰─ーーーーー─┴────────┴───────╯

@@ -8 +8,2 @@
-<keb>憤まん</keb>
+<keb>忿懣</keb>
+<ke_inf>&rK;</ke_inf>
@@ -11 +12,2 @@
-<keb>忿懣</keb>
+<keb>憤まん</keb>
+<ke_inf>&sK;</ke_inf>
@@ -14,0 +17 @@
+<ke_inf>&sK;</ke_inf>
@@ -21 +23,0 @@
-<pos>&vs;</pos>