Why this linear to db function works only when given 2 polarization bands?

Question

I am working only on VV polarization of Sentinel-1 Images. Wanted to use the function from other answer (Convert Sentinel-1 images data from dB to linear) but it doesn't seem to work when given only VV. Why is that? How do I make it work when having only 1 band? Please be aware that originally image has 3 bands, so I really don't understand that behaviour. Here's the test code:

var filter = ee.Filter.and(
  ee.Filter.bounds(Map.getBounds(true)),
  ee.Filter.date('2022-01-01', '2022-02-01')
)

var db = ee.ImageCollection('COPERNICUS/S1_GRD')
  .filter(filter)
  .mosaic()

var linear = ee.ImageCollection('COPERNICUS/S1_GRD_FLOAT')
  .filter(filter)
  .mosaic()

    
Map.addLayer(db, {bands: 'VV', min: -25, max: 0}, 'DB')
Map.addLayer(linear, {bands: 'VV', min: -25, max: 0}, 'LINEAR')  
Map.addLayer(toDb1(linear), {bands: 'VV', min: -25, max: 0}, 'AFTER DB1') // THIS WORKS
Map.addLayer(toDb2(linear), {bands: 'VV', min: -25, max: 0}, 'AFTER DB2') // THIS DOES NOT

  

function toDb1(linear) {
  return linear.addBands(
    ee.Image().expression('10 * log10(linear)', {
      linear: linear.select(['VV', 'VH']) // VV and VH
    }),
    null, true // Replace the bands to keep image properties
  )
}

function toDb2(linear) {
  return linear.addBands(
    ee.Image().expression('10 * log10(linear)', {
      linear: linear.select(['VV']) // only VV
    }),
    null, true // Replace the bands to keep image properties
  )
}

Link to the code: https://code.earthengine.google.com/f3656ddfd179f7559c7425e96e913faf

Answer 1

This is expression() behaving a bit inconsistent. In the multi-band case, your resulting bands are named after the input. When you have a single band, your result is constant. Manually rename the output and you're good.

  return linear.addBands(
    ee.Image().expression('10 * log10(linear)', {
      linear: linear.select(['VV'])
    }).rename('VV'),
    null, true // Replace the bands to keep image properties
  )

https://code.earthengine.google.com/388031402ec8e54e6161bde02cc108a8